1. Field of the Invention
This invention relates to the field of automated assembly, and in particular to the field of printed circuit board component placement and assembly.
2. Description of Related Art
Printed circuit boards are typically assembled via the use of component placement machines, typically called "pick and place" machines. Electronic components are automatically placed on the printed circuit board by a placement machine and subsequently electrically attached to the board, using, for example, wave soldering techniques. Typically, the component placement machine comprises multiple component placement modules, so that multiple components can by placed on the printed circuit board at the same time. For mechanical efficiency, component placement machines or modules have a limited range of travel for placing the components, and therefore, the printed circuit board is typically processed in steps, each step placing a different portion of the printed circuit board within the range of the placement machine or module.
FIG. 1 illustrates a top view of an example assembly line of printed circuit boards 100 (100A, 100B, 100C, 100D, and 100E) and a component placement machine that includes eight component placement modules 150 (150A-150H). Each placement module 150 includes a feeder bar 160 that supplies components (not shown) to a movable head 170. The feeder bar 160 includes feeder slots 162 through which each component is supplied. The movable head 170 subsequently places the component at its appropriate location (x, y) on the printed circuit board 100 that is within its range. The movable head 170 travels in a latitudinal (y) direction on a movable bar 175 that moves in a longitudinal (x) direction to reach the appropriate location (x, y), and then moves in a vertical (z) direction (perpendicular to the view of FIG. 1) to place the component on the printed circuit board 100. The movable head 170 also rotates within the x-y plane, so that components may be placed in different orientations on the printed circuit board 100. After each of the placement modules 150 place their allocated components, the printed circuit boards 100 move a predetermined distance, traveling for example on a conveyor system 120. After moving the predetermined distance, the placement modules 150 place the next set of allocated components, the allocation being determined based upon the area of the printed circuit board 100 that is within the reach of each placement module 150.
As is commonly known in the art, the speed with which the component placement machine can populate each board 100 is a function of the particular components allocated to each particular placement module 150, and the order in which each placement module 150 obtains these selected components. Ideally, for example, one would want all of the placement modules to complete their placements of allocated components at exactly the same time at each step, thereby avoiding periods of inefficiency when only a few of the heads are being utilized. Other constraints also limit the speed of operation, or throughput, of the component placement machine. For example, the number and type of feeder slots 162 provided to each feeder 160 may limit the choice of components that may be allocated to a particular placement module.
The allocation of components to placement modules is a combinatorially complex problem having no known closed-form solution. Conventionally, the allocation is performed manually, or via the use of conventional programs based upon, for example, heuristics algorithms. U.S. Pat. No. 5,390,283, "Method for Optimizing the Configuration of a Pick and Place Machine", by Larry J. Eshelman and James D. Schaffer, issued Feb. 14, 1995, presents a method for determining a near-optimal allocation of components to placement modules through the use of an evolutionary algorithm, and is incorporated herein by reference. In that patent, as in the conventional method of component allocation, the step size is predetermined for a given placement machine, based upon the pitch of the printed circuit boards on the conveyor system. The pitch is defined as the distance from the start of one printed circuit board to the start of the next printed circuit board; it is the width of the printed circuit board plus the space between boards on the conveyor system. A stepping scheme for a printed circuit board with a pitch of 360 mm, for example, is presented in the referenced patent as a sequence of a 40 mm step followed by four 80 mm steps.
As discussed above, each step movement of the conveyor system determines the area of each printed circuit board that is within the reach of each placement module. As such, the choice of step movements will have a direct effect on the allocation options provided for each module, and therefore a direct effect on the speed of the component placement process. The choice of step movements, however, has a combinatorial effect on the allocation process, which is already a combinatorially complex problem. As such, the determination of a step movement that provides for optimal or near optimal throughput from the component placement machine does not currently have a closed form solution. For example, the aforementioned pitch of 360 mm could be processed as six steps of 60 mm each, rather than the steps of 40 mm followed by four 80 mm presented above. There will be, for example, a delay time associated with the starting, moving, and halting of the conveyor system for the additional sixth step, against which any gains of speed by different allocations must be weighed. In like manner, the pitch of 360 mm could be processed as nine 40 mm steps, three 120 mm steps, and so on. Even if the number of steps is the same, the choice of step size can affect the allocation; for example, a step of 80 mm followed by four steps of 70 mm might allow for a faster process than the aforementioned 40 mm followed by four 80 mm steps. Each choice of step sizes produces a substantially different set of options to the allocation process, dependent upon the relationships of the components being placed in the different areas within range of each movable head at each step.